It is known in the field of printing that inkjet printing heads require periodic purging to clean the printing nozzles, remove air bubbles, and maintain printing quality. Simply stated, inkjet printers operate by expelling a small volume of ink from a plurality of nozzles through corresponding small orifices in a nozzle plate held in proximity to a paper or front surface of a solar cell or other medium upon which printing or marks are to be placed. These orifices are arranged in a fashion in the nozzle plate such that the expulsion of droplets of ink from a selected number of nozzles relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the medium followed by another expulsion of ink droplets results in the creation of more segments of the desired character or image.
A solar cell is a solid state semiconductor device that converts the energy of sunlight directly into electricity. Most large-scale commercial solar cell factories make printed poly-crystalline silicon solar cells. The printed poly-crystalline silicon solar cells have grid-like metal contacts made up of fine fingers and larger bus bars printed onto the front surface using a silver paste inkjet printing. The grid-like metal contact is made of 60 to 100 printed silver lines, 2 millimeter (mm) apart and having a typical width of 100 micrometers (um).
The frequency of purging the printing head depends on the specific application for which the printing head is being used. Purging includes forcefully pushing a printing liquid out of the inkjet head through the nozzles under an excess pressure of roughly 0.5 Bar. The force of the excess pressure is relative to a normal reduced pressure used during printing of about −0.01 Bar.
One of the disadvantages of conventional purging techniques is that a printing head needs to be shifted from the printing area to a maintenance area where the purged liquid is able to drip from the head, and any liquid that drips from the head will not adversely affect the printing process. A nozzle plate, as is generally known in the industry, is located on the printing side of the printing head, providing access for the nozzles to print while providing protection for the printing head, among other features.
After purging, it is preferable for a nozzle plate to be cleaned, known as wiping, to remove purged liquid and enable proper jetting of the printing liquid from the nozzles. In order to preserve the smoothness and anti-wetting characteristic of the nozzle plate, it is desirable to perform wiping without contact to the nozzle plate.
One conventional technique for wiping without contact to the nozzle plate is vacuum wiping, where a vacuum head is moved across the nozzle plate. The vacuum head does not contact the nozzle plate but is sufficiently close to allow the vacuum, also known as suction, to remove the purged liquid from the nozzle plate. As the vacuum head does not contact the nozzle plate, there is suction from all sides of the vacuum head (not just from the direction of the nozzle plate) resulting in low cleaning efficiency of the nozzle plate.
Disadvantages to conventional vacuum wiping include cost, printing speed, reliability, and quality of wiping. Examples of the disadvantages of conventional vacuum wiping include: (a) The printing head requires shifting from the printing area to a maintenance area due to the possibility of purged liquid dripping from the printing head, (b) Shifting a printing head from a printing area to a maintenance area takes time, resulting in reduced printing speed and correspondingly increased cost. (c) The printing machine requires very high precision elements and structure to enable a vacuum slit to move across the nozzle plate at a small distance without contact, for example about 0.15 mm. Higher precision elements increase the cost of a printing machine, and the reliability of wiping is dependent on maintaining high precision, (d) Effective wiping requires the vacuum slit to move across the nozzle plate at a slow speed, for example about 2 mm/s (millimeters per second). The time required for effective wiping reduces printing speed and correspondingly increases printing cost.
There is therefore a need for a printing system that may be used for printing a solar cell grid-like metal contact for example that enables purging without purged liquid dripping from the nozzle plate (or in general from the printing head). It is further desirable for high quality cleaning with lower costs and higher reliability than current techniques. Furthermore, there is a need to prevent the formation of sediment on the nozzle plate and for removing part of the fumes accumulated on the nozzle surface to prevent deflection of the printed ink jets.